The present invention relates to a lock-up device for a torque converter.
A torque convertor is a device having a tubular toroid composed of three types of vane wheels (an impeller, a turbine, and a stator) for transmitting power by a hydraulic fluid contained therein. The impeller is fixed to a front cover connected to an input-side rotation member. The turbine driven by the hydraulic fluid flowing in from the impeller is connected to an output-side member.
There has been a torque converter in which a lock-up device is disposed between a turbine and a front cover. Such a lock-up device is for directly transmitting torque to an output-side member from the front cover. The lock-up device generally comprises a piston connected to tile output-side member such as a turbine shell. A frictional member is fixed to the side surface of the piston. If this frictional member abuts on the frictional surface of the front cover, torque produced by tile front cover is mechanically transmitted to tile output-side member through the piston.
In the above described conventional case, the lock-up device has only one frictional surface. If engine torque is increased, the torque transmission capacity of the lock-up device may, in some cases, be insufficient.
In order to increase the torque transmission capacity of the lock-up device, it is considered that the area of the frictional surface between the piston and the front cover is increased. In this case, however, it is necessary to increase the outer diameter of the front cover. Consequently, the size of the torque converter is increased In the radial direction.
Furthermore, an attempt has been made to increase the torque transmission capacity by employing a multiple disk clutch for the lock-up device. In the multiple disk clutch, however, at least more than two plates must be arranged between the piston and the front cover. Consequently, the size of the torque converter is increased in the axial direction.